 It's not a plantation, so what people would want slaves for would be a little bit different than, say, the colonies of Georgia or Virginia. But still, hard labor is hard labor. The captain didn't really probably think anyone was going to be interested in Phyllis at the time because she was really small and frail and dirty and on the brink of death. In fact, actually, she might have most likely suffered breathing afflictions for the rest of her life due to the conditions of the ship that were forced on her. I imagine maybe something close to asthma.